Method of heat treating high-carbon corrosion resistant steels



Aug. 15, 1967 Filed May 2-2, 1964 VPN R. KIHLBERG E L 3,336,169

METHOD OF HEAT TREATING HIGH-OARBON CORROSION RESISTANT STEELS 2Sheets-Sheet 1 10b 20'0 360 460 560 e60 760 c INVE TORS Ragnar l rhL erErlk G-unnmr EclLL en BY R. l (lHLBERG ETAL METHOD OF HEAT TREATINGHIGH-CARBON CORROSION RESISTANT STEELS Filed May 22, 1964 VPN 2Sheets-Sheet 2 INVENTOR-S Ra. new" l lhlber Eri Gunmu' EdL/ an BY PM 2go M United States Patent 3,336,169 METHOD OF HEAT TREATING HIGH-CARBONCORROSION RESISTANT STEELS Ragnar Kihlberg and Erik Gunnar Edliden,Munkfors,

Sweden, assignors to Uddeholms Aktiebolag, Uddeholms, Sweden, a Swedishjoint-stock company, and Wilkinson Sword Limited, London, England, aBritish joint-stock company Filed May 22, 1964, Ser. No. 369,535 Claimspriority, application Sweden, May 28, 1963, 5,910/ 63 1 Claim. (Cl.148-125) This invention relates to a method of heat treating high-carboncorrosionresistant steels adapted for manufacturing fine cutting toolsand instruments, for example razor blades, intended for cuttingrelatively soft materials (thus, not for metal working), said methodcomprising the steps of first heating the steel to a temperature aboveits top temperature (the lowest temperature to which the steel must beheated for obtaining maximum hardness after cooling to room temperature)whereafter the steel is cooled to room temperature or possibly a lowertemperature and, finally, heated again.

The invention relates to a method of heat treating the steel and furtherto steel, cutting tools, such as razor blades and blanks (hardened butnot ground blades) and to steel strips for such cutting tools treated(manufactured) according to the invention.

The invention is particularly adapted for use in the mass production ofrazor blades and other similar cutting tools having sharp edges (but notfor metal cutting tools). The invention has as one of its objects toobtain for high-carbon corrosion resistant steels substantial hardnessand to maintain this hardness even in the cutting tool after itsmanufacture.

The term hardness is to be understood to be the property of the materialwhich is determined by a hardness measuring instrument according to theVickers method and expressed by the relation between the impression loadand the impression area.

In a method previously known through Swedish Patent No. 126,577 thesteel is heated to a temperature which is from 20 to 50 C. higher thanthe top temperature and reheated to a temperature above 30 C. but notexceeding 200 C.

Steel of this type was originally hardened from temperatures of about1025 C. When higher temperatures were used, the amounts of retainedaustenite obtained were so large that the hardness was reduced. Byapplying sub-zero treatment (Molinder hardening) the hardeningtemperature could be increased to 1075-l090 C. whereby more carbideswere dissolved. The increased amount of retained austenite wastransformed by said sub-zero treatment to martensite whereby a higherhardness of the hardened steel could be obtained. At the present time,the hardening is carried out from a relatively high temperature and isfollowed by sub-zero treatment and tempering at low temperature. Herebya hardness of about 800 H, is obtained.

It was found, however, that this hardness of the hardened strip was notmaintained, for example, in the razor blade after its manufacture. Thisis partly due to the fact that modern razor blade production includes inmany cases processes wherein the razor blade is heated to relativelyhigh temperatures which result in a substantial decrease in hardness ofthe entire blade. As a further and main reason is to be mentioned thatgrinding results in such a local heating of the outermost edge that saidedge is tempered to a relatively low hardness. As the outermost portionof the edge constitutes the active portion of the razor blade, theadvantage gained by the higher starting of the hardened strip, thus, islost.

The invention is substantially characterized in that the steel is heatedto such a high temperature, preferably from 50 to C. higher than its toptemperature, that during the cooling relatively large amounts ofaustenite are retained, and that the steel is reheated to a temperaturebetween 400 and 700 C. whereby a significant increase in hardness isobtained.

The hardness obtained in this way is both higher than the hardness ofthe untempered steel and, above all, considerably higher than thehardness of steel of the same analysis hardened in a conventional manner(by normal hardening or sub-zero treatment) which was tempered to thesame temperature.

It is probable, but not proved that the reheating (tempering) of thehardened steel which brings about a significant increase in hardness,depends on a precipitation of alloying elements in the austenite phase.The alloying content in the austenite phase is thereby reduced, so thatduring the subsequent cooling the austenite can easily be transformed tomartensite. In order to be able to utilize this effect, it is importantto carry out the heating to such a high temperature that more carbidesare dissolved than by normal hardening, whereby, during the coolingrelatively large amounts of austenite are retained.

The high-carbon corrosion resistant steel to be treated according to theinvention contains 0.8 to 1.4%, preferably 0.9 to 1.1% carbon 11 to 16%,preferably 12.5 to 14.5% chromium and possibly impurities and/ oradditions of sulphur, phosphorus, manganese, silicon, vanadium,molybdenum, tungsten, antimony, nitrogen, cobalt, nickel, aluminum andtitanium in an amount smaller than 5%, preferably smaller than 2,5%.

The steel is preferably heated to a temperature between 1100 and 1250C., preferably between 1125 and 1 C.

According to the preferred embodiment of the invention the coolingcomprises the steps of quenching, preferably in oil or between coolingplates, possibly followed by a sub-zero treatment, preferably in DryIce, to a temperature between 50 C. and 80 C., the reheating beingcarried out to a temperature between 450 and 650 0., preferably between500 and 600 C.

The invention is described below by way of examples, reference being hadto the accompanying drawings wherein:

FIGS. 1 and 2 show the hardness as a function of the temperingtemperature for steel hardened at high temperature.

Example 1 A steel having the analysis Approximate percent 1 S1 0.2 Mn 1Cr 14 in the form of a strip with a thickness of 0.90 mm. is annealedand austenitized at a temperature of 1150" C. in a tube-type furnace.The total heating time was 10 minutes. The cooling was carried out byquenching in oil followed by sub-zero treatment in Dry Ice at about 70C.

The tempering was carried out at different temperatures in a convectionfurnace for 30 minutes.

The result obtained is shown in FIG. 1. As appears therefrom it isposible to obtain a hardness of about 740 V.P.N. by an austenitizingtemperature of 1150 C. with subsequent cooling and tempering at 550 C.

Example 2 Punched razor blades having a thickness of 0.10 mm. and theanalysis Approximate percent C 1 Si 0.2 Mn 1 Cr 13.5

were austenitized at 1125 C., 1150 C. and 1175 C. in a tube-typefurnace. The total heating time was 3 minutes. The cooling comprisedquenching between cooling plates and subzero treatment in Dry Ice atabout 70 C. The tempering was carried out at different temperatures in aconvection furnace for 30 minutes.

The results are shown in FIG. 2. Even an austenitizing temperature aslow as 1125 C. appears to be sufficient for rendering possible aprecipitation which increases the hardness to 725 V.P.N. Under thesetrial conditions the tempering temperature 525 C. resulted inmaximumhardness. The austenitizing temperature 1150 C. produced a hardness of720 V.P.N. at the tempering temperature 550 C. The austenitizingtemperature 1175 C. resulted in 690 V.P.N. at a tempering temperature ofabout 575 C.

In FIGS. 1 and 2 certain differences between the curves can be observed,which is due to the difference in the austenitizing time which resultsin different amounts of being dissolved at the same temperature, and tosome extent also to the difference in the thickness of the material.

Example 3 ments. The blades had the punching burrs during the I,

breaking operation on their concave side.

The Test Nos. 1, 2 and 3 as well as the test in untempered state werenot carried out according to the invention, but are only included forcomparison.

TABLE 1 TABLE 2 After the first tempering hardness V.P.N.

After the second ternpering hardness V.P.N.

Razor blade no.

The above tests have proved (a) that steel treated according to theinvention maintains its high hardness at tempering temperatures at whichsteel hardened in a normal way had already become substantially softer(see FIG. 1), (b) that the increase in hardness probably does notsubstantially depend on the new formation of martensite from retainedaustenite during the cooling after the first tempering, but probably onsome sort of precipitation hardening.

What we claim is:

A method of heat treating a high-carbon corrosive resistant steelintended for manufacturing cutting instruments adapted for cuttingrelatively soft material, said steel containing from 0.8 to 1.4% carbon,from 11 to 16% of chromium and less than 5% of the elements sulfur,phosphorus, manganese, silicon, vanadium, molybdenum, tungsten,antimony, nitrogen, cobalt, nickel, aluminum and titanium, the remainderbeing substantially all iron, comprising the steps of first heating thesteel to a temperature of from 1100 to 1250 C., for at least about 3minutes, quenching the heated steel to a temperature of from -50 to -80C. and then reheating the steel to a temperature of from 450 to 650 C.for at least about 30 minutes.

References Cited Alloy Digest, Filing Code SS-101, Stainless Steel,March 1960, Engineering Alloy Digest Inc., Upper Montclair, NJ.

Republic Stainless Steels, 1951, Republic Steel Corp, Cleveland, Ohio,relied on pages 58 to 52.

Steel and Its Heat Treatment, vol. I, 1948 Bullens, John Wiley & Sons,Inc., N.Y., relied on pages 380-385, page 256.

Steel and Its Heat Treatment, vol. III, 1949 Bullens, John Wiley & Sons,Inc., N.Y., relied on pages 573-578.

Test No. Austenitizlng Not tempered Tempered Tempered temp., 0. 300 0.500 C.

Tempered 525 C.

Tempere Example 4 For finding out how the hardness changes as a resultof repeated heating a so-called double tempering was car- 70 DAVID L.RECK, Primary Examiner.

HYLAND BIZOT, Examiner.

C. N. LOVELL, Assistant Examiner.

